FIG. 1 illustrates an example of a wireless communications network that includes one or more wireless devices 110 (which may interchangeably be referred to as user equipment, UEs) and one or more network nodes, such as wireless network nodes 120 (e.g., a base station or an evolved Node B, eNB) and core network nodes 130. Wireless network nodes 120 can be associated with various types of cells, such as legacy cell 120a (e.g., a cell configured to transmit at least one type of reference signal in each subframe over a time period, T0) and on/off cell 120b (e.g., a cell that does not transmit any type of reference signal in at least one subframe over the time period, T0). In general, wireless devices 110 within coverage of a wireless network node 120 communicate with the wireless network node 120 over a wireless interface. For example, wireless devices 110 and wireless network nodes 120 may communicate wireless signals containing voice traffic, data traffic, and control signals. Core network node 130 manages the establishment of communication sessions and various other functionality for wireless device 110. The network nodes connect through interconnecting network 125, which refers to any interconnecting system capable of transmitting audio, video, signals, data, messages, or any combination of the preceding.
Small Cell on/Off
In densely deployed small cells, it is necessary to ensure low interference between cells for ensuring efficient operation. One of the mechanisms for interference avoidance and coordination among small cells is small cell on/off feature. According to this feature the small cell may be turned on and off where the “on” and “off” period may depend on the criteria or application.
In semi-static small cell on/off, the criteria for cell on/off can be traffic load, UE arrival/departure, etc. On the other hand, in dynamic small cell on/off, the small cell can be turned on and off on radio frame level or even subframe level. The criteria in this case can be packet arrival/completion or interference coordination and avoidance (i.e. reduce interference towards other nodes or UEs). So this means that the cell turns off at the subframe boundary (or end of current subframe) when the transmission of packet is completed and turns on at the next subframe boundary when a new packet arrives.
Another purpose of small cell on/off can be for energy saving. Some preliminary evaluation of the energy saving impact of the small cell on/off is presented in 3GPP TR 36.872, ver. 12.0.0, “Small cell enhancements for E-UTRA and E-UTRAN; Physical layer aspects.” Some discussion of physical layer aspects of small cell enhancements is presented in 3GPP RP-132073.
There are different operational modes of small cell on/off                Handover: In this mode a UE in CONNECTED mode is always attached to a cell. Due to increased traffic demand, for example, the network may decide to offload the UE by handover to a small cell. The small cell can be “off” and can wake up to serve the UE. The handover time in this case depends on the backhaul delay and the handover execution time. After completion of the transmission and/or reception of data, the UE goes to IDLE mode or is handed over to another cell and the small cell can be turned off again.        SCell only: In this mode a carrier aggregation (CA) capable UE is connected to a primary cell (PCell), and the network configures a secondary cell (SCell) that can be turned on or off. If the network decides to offload the UE traffic to SCell, then the SCell is turned on.        Dual connectivity: In this mode the UE is connected to two network nodes (or two or more cells from different network nodes), and one of the nodes (or one or more of the cells) can be turned on and off.        Serving cell: In this mode a cell can be either on or off when a UE is connected to it. Certain procedures for radio resource management (RRM), radio link monitoring (RLM), and channel state information (CSI) measurements must be designed for this case.        
Discovery Signal
In small cell on/off where the enhanced node B (eNB) can be off for a long period of time, a discovery signal might be needed in order to assist the UE with the measurements. This is referred to as Discovery Reference Signal (DRS) in some cases. The discovery signal needs to support the properties for enabling RRM measurements, RLM related procedures, and coarse time/frequency synchronization. In order to make the measurements possible, the eNB has to wake up periodically (e.g. once every 40 ms, 80 ms, or 160 ms, etc.) and send the discovery signal so that it can be used by the UE for mobility related operations such as cell identification, RLM, and measurement.
Since the discovery signal is rather sparse in time, it is desirable that the UE is able to make a meaningful measurement in one instance of the discovery signal rather than having to wait for multiple instances which may occur tens or hundreds of milliseconds apart. In addition to that, in order to make the measurement based on fewer samples in time more reliable, the discovery signal may need to be sent on wide bandwidth (e.g. the whole bandwidth).
It has been agreed in 3GPP that UE assumes primary synchronization signal (PSS)/secondary synchronization signal (SSS)/cell-specific reference signal (CRS) in the discovery reference signal (DRS). (See 3GPP TR 36.872, ver. 12.0.0, “Small cell enhancements for E-UTRA and E-UTRAN; Physical layer aspects.”). Additionally Channel State Information-Reference Signals (CSI-RS) is assumed in the DRS for measurement if configured by higher layers. Both CRS-based reference signal received power (RSRP) measurements and CSI-RS-based RSRP measurements are supported. UE may report DRS-based RSRP/RSRQ (reference signal received quality) and associated physical cell identity (PCID) and information for TP identification.
Furthermore, for DRS-based measurement, a UE assumes that a DRS occasion for a cell consists of                One instance of PSS/SSS per Rel-8        CRS is transmitted at least in the same subframe(s) as PSS/SSS        A DRS occasion can comprise multiple CSI-RS RE configurations. The different CSI-RS configurations may be in the same or different subframe(s)        
A DRS occasion for a cell comprises of N consecutive subframes (N<=5), and DRS occasion for a cell occurs every M ms (candidate values for M so far are 40, 80, 160 ms).
Examples of Radio Measurements
Several radio-related measurements are used by the UE or the radio network node to establish and keep the connection, as well as ensuring the quality of a radio link.
The measurements are used in radio resource control (RRC) idle state operations such as cell selection, cell reselection (e.g. between E-UTRANs, between different radio access technologies (RATs), and to non-3GPP RATs), and minimization of drive test (MDT), and also in RRC connected state operations such as for cell change (e.g. handover between E-UTRANs, handover between different RATs, and handover to non-3GPP RATs).
The UE has to first detect a cell and therefore cell identification, e.g. acquisition of a physical cell identity (PCI), is also a signal measurement. The UE may also have to acquire the cell global ID (CGI) of a node (or cell).
The RSRP and RSRQ are used for at least RRM measurements such as for mobility, which include mobility in RRC connected state as well as in RRC idle state. The RSRP and RSRQ are also used for other purposes, e.g. for enhanced cell ID positioning, minimization of drive test etc.
In RRC connected state the UE can perform intra-frequency measurements without measurement gaps. However as a general rule the UE performs inter-frequency and inter-RAT measurements in measurement gaps unless it is capable of performing them without gaps. To enable inter-frequency and inter-RAT measurements for the UE requiring gaps, the network has to configure the measurement gaps. Two periodic measurement gap patterns both with a measurement gap length of 6 ms are defined for LTE:                Measurement gap pattern #0 with repetition period 40 ms        Measurement gap pattern #1 with repetition period 80 ms        
The measurements performed by the UE are then reported to the network, which may use them for various tasks.
The radio network node (e.g., base station) may also perform signal measurements. Examples of radio network node measurements in LTE are propagation delay between UE and itself, uplink signal-to-interference-plus-noise ratio (UL SINR), uplink signal-to-noise ratio (UL SNR), uplink signal strength, Received Interference Power (RIP), etc. The eNB may also perform positioning measurements which are described in a later section.
The UE also performs measurements on the serving cell (aka primary cell) in order to monitor the serving cell performance. This is called as radio link monitoring (RLM) or RLM related measurements in LTE. For RLM the UE monitors the downlink link quality based on the cell-specific reference signal in order to detect the downlink radio link quality of the serving or PCell.
In order to detect out-of-sync and in-sync, the UE compares the estimated quality with the thresholds Qout and Qin respectively. The threshold Qout and Qin are defined as the level at which the downlink radio link cannot be reliably received and corresponds to 10% and 2% block error rate of a hypothetical PDCCH transmissions respectively.
Sampling of Measurement
The overall serving cell or neighbour cell measurement quantity results comprises of non-coherent averaging of 2 or more basic non-coherent averaged samples. The exact sampling depends upon the implementation and is generally not specified. An example of RSRP measurement averaging in E-UTRAN is shown in FIG. 2. The figure illustrates that the UE obtains the overall measurement quantity result by collecting four non-coherent averaged samples or snapshots (each of 3 ms length in this example) during the physical layer measurement period (i.e., 200 ms) when no discontinuous reception (DRX) is used or when DRX cycle is not larger than 40 ms. Every coherent averaged sample is 1 ms long. The sampling also depends upon the length of the DRX cycle. For example for DRX cycle >40 ms, the UE typically takes one sample every DRX cycle over the measurement period.
A similar measurement sampling mechanism is used for other signal measurements by the UE and also by the base station (BS) for UL measurements.